28 L. Leigh Fermor — Laterites of French Guinea. 



V. — The Work of Professor Lacroix on the Laterites of 

 French Guinea. 



By L. Leigh Fermor, D.Sc, A.B.S.M., F.G.S., Geological Survey of India. 



Contents. 



Part I. 

 I. Introduction. 

 II. The Nomenclature of Laterite. 



III. The Minerals of Laterite. 



IV. The Processes and Products of Lateritization. 

 IV (a). The Zone of Leaching {Zone d& depart). 



Gibbsitic Laterites. 



Ferruginous Laterites. 



Kaolinic and Argillaceous Decomposition. 



Part II. 



IV (b). The Zone of Concretion (Zone de concretion). 

 Gibbsitic Types. 

 Ferruginous Types. 

 Bauxitic Types. 

 Pisolitic Laterites. 

 The Hardening of Laterites. 

 Alluvial Laterites or Lateritites. 

 Lateritized Alluvium (Lat6rites d' alluvions). 

 V. The Conditions of Formation of Laterite. 

 VI. The Age of Laterites. 



I. Introduction. 



A SHORT while ago I received from Professor A. Lacroix a copy 

 of a recently published memoir on the laterites of French 

 Guinea, accompanied by the request that I should, if possible, give an 

 account of this work in the Geological Magazine, " puisque c'est la. 

 qu'ont paru les articles les plus importants sur cette question." 

 I have undertaken this task with the greater pleasure because 

 Professor Lacroix was so good as to show me, when I was passing 

 through Paris recently, both his hand-specimens and his microscopic 

 preparations of these rocks, and because the memoir in question seems 

 to me to be one of the most important and thorough pieces of work on 

 laterite published since Max Bauer's account of the laterite of the 

 Seychelles. 1 Although Professor Lacroix' memoir deals professedly 

 with French Guinea only, yet it is probable that many of the 

 conclusions will be found to apply to other regions, and on this 

 account, and because the series in which the memoir appears will 

 probably not be readily accessible to all, I have thought it desirable 

 to expound it at some length. 



The full title of the memoir is " Les laterites de la Guinee et les 

 produits d' alteration qui leur sont associes ", the medium of publication 

 being the Nouvelles Archives du Musee, ser. v, torn, v, pp. 255-356, 

 with pis. x-xvii, 1914.. It is the fruit of a visit to West Africa 

 by the author in the winter of 1912-13, when he was able not only to 

 explore in detail the Archipelago of Los, but also to traverse many 

 parts of French Guinea. From Konakri on the coast to the Niger 



1 Neues Jahrb.filr Min., etc., ii, pp. 192-219, 1898. 



